1. Field of the Invention
The present invention relates to an apparatus for removing harmful components in an exhaust gas derived during manufacturing an electronic circuit element such as a semiconductor device or a liquid crystal device, particularly during a cleaning or etching process. Further, the present invention relates to an apparatus that is applicable to remove harmful components in a gas generated during the process of smelting aluminum.
2. Description of the Prior Art
In a semiconductor manufacturing apparatus such as a CVD apparatus, a gas for deposition (such as SiH.sub.4, Si.sub.2 H.sub.6, SiH.sub.2 Cl.sub.2, TEOS, PH.sub.3, B.sub.2 H.sub.6, NH.sub.3, N.sub.2 O, or the like) is used in forming various thin films, and a cleaning gas (such as NF.sub.3, SF.sub.6, C.sub.2 F.sub.6, CF.sub.4, or the like) is usually used for cleaning the inside of the semiconductor manufacturing apparatus after completing the deposition process.
These gases inherently have various dangerous factors such as flammability, explosiveness, corrosiveness, poisonousness, and the like. Therefore, it is required to remove (detoxify) harmful components in these gases using a harm-removing apparatus equipped, for example, with a means for oxidizing and heating the gases before they are released into atmospheric air.
In the semiconductor manufacturing apparatus such as a CVD apparatus, complex decomposition reactions occur in the gases that are being used, so that new decomposition products (such as F.sub.2, HF, HCl and SiOx) are generated as a result and these decomposition products are discharged together with the undecomposed deposit gas and the cleaning gas.
Here, one group of gases to be used is what is known as PFC gases. This is an abbreviation for "perfluorocarbon" or "perfluoride compound". Representative examples of PFC are CF.sub.4, CHF.sub.3, and C.sub.2 F.sub.6. If the term "compound" is used instead of "carbon", PFC will further include fluorine compounds that do not contain carbon, such as NF.sub.3, SF.sub.6, and SF.sub.4.
The purpose of the present invention is to establish a technique for removal of the former PFC gases, which technique has not yet reached an applicable level for use in a removal device or a removal method. Naturally, however, the technique of the present invention is also applicable for removal of all the PFC gases including the latter PFC gases.
The PFC gases as represented by CF.sub.4 and C.sub.2 F.sub.6 are non-flammable and toxicity of the gases themselves on human beings is unknown. At least, acute and subacute toxicities are not known. However, since the compounds themselves are stable, they remain unchanged and stay for a long period of time if they are released to an ambient atmosphere. It is reported that the life span until consumption in the atmosphere is 50,000 years in the case of CF.sub.4, and 10,000 years in the case of C.sub.2 F.sub.6.
Further, the global warming factor (relative to CO.sub.2) is 4,400 in the case of CF.sub.4 and 6,200 in the case of C.sub.2 F.sub.6 (after 20 years have passed), which involves a problem that cannot be left aside in considering the earth environment. Therefore, it is eagerly desired to establish a means for removing the PFC gases as represented by CF.sub.4 and C.sub.2 F.sub.6.
However, the former PFC gases, i.e. the compounds as represented by CF.sub.4, CHF.sub.3, and C.sub.2 F.sub.6, have a stable C--F bond (having a bonding energy as large as 120 kcal/mol) and are not easily decomposed, so that it is extremely difficult to remove them by simple thermal oxidation decomposition.
For example, in the case of C.sub.2 F.sub.6, the decomposition proceeds by a cut in the C--C bond, so that C.sub.2 F.sub.6 can be removed by limiting the processing flow rate to be less than 250 liters/min at a processing temperature of 1000.degree. C. However, in the case of CF.sub.4, it is necessary to cut the C--F bond that has the largest bonding energy, so that a temperature of 1400.degree. C. is required even with the above-mentioned flow rate. In addition, even by the above method, it is difficult to remove more than 80% of the total gas.
Further, if an electric heater is to be used, attainment of a high temperature atmosphere of more than 1400.degree. C. is an upper limit also from the view point of materials for the heater, so that a long-term usage is almost impossible. Moreover, maintaining the temperature of the entire apparatus is also difficult and, in combination with a thermal insulating material, the total volume of the apparatus will be large and it will not be a compact apparatus. What is more important is that the thermal energy cost will be excessively high.
Here, in this field, the following new method has been proposed. International Publication Number WO94/05399 #Method of Decomposing Gaseous Halocarbon# reports that coexistence of O.sub.2 makes it possible to decompose and remove, for example, CF.sub.4 at a temperature of 600 to 700.degree. C. However, a detailed follow-up experiment of the contents of the publication turned out to be a complete failure in removal under this condition.
Also, an attempt is made to positively introduce H.sub.2 gas to pyrolyze PFC. However, it requires a high processing temperature and, besides, it may not be suitable for use from the view point of safety, since the H.sub.2 gas is flammable and explosive.